Monitoring physiological variables such as pressure, pH, oxygen level and glucose is useful for routine biomedical research and clinical practice. Patients in critical care units are typically monitored at regular intervals, and often continuously, so that medical staff can track their physiologic status in real time. However, continuous monitoring of these physiological parameters often requires tethered, wired connections. For example, use of an implanted catheter with an external transducer is standard for monitoring intracranial pressure (ICP) in patients suffering from head trauma. The ability to assess intracranial hypertension continuously is important for prompting intervention and achieving favorable outcomes. However, tethered solutions cause patient discomfort and carry a risk of infection and complications stemming from dislodgement, leakage and blockage.
Passive (non-tethered) solutions have included low frequency approaches, which may require relatively large components and translate to a sensor size in the cm3 range. Wireless solutions, such as those for ICP monitoring, may also involve large sizes, as they use batteries and active circuitry to power the sensor device. Further, while integrated circuit (IC) chip size can be relatively small for actively powered wireless pressure devices, these devices generally require additional space for a separate antenna coil. Further, certain passive strategies have been limited by self-resonant frequencies of readout circuitry, as interference effects make it difficult to detect sensors operating near and above this frequency. These and other matters have presented challenges to various sensor applications.